(a) Field of the Invention
This application relates to a system and method for controlling the speed of the blades of a wind turbine. More particularly, but not by way of limitation, to a wind turbine that includes blades that capture air as the blades rotate, and use this air to operate ram air turbines that assist in slowing the speed of the blades, while generating electrical power.
(b) Discussion of Known Art
Current trend in the design of wind power turbines for generation of electricity is towards longer blades. This allows a greater amount of torque to be applied at a lower wind speed but it does have several drawbacks inherent to the concept. Longer blades require taller towers which in turn require stronger materials and better base preparations. Such tall towers are limited in geographic locations where they can be placed due to soil conditions, etc. without much greater added expense. Once the taller tower is installed the problems are not over as the longer blades are more wind sensitive. Higher wind speeds are required for start up yet the long blades must be shut down at much lower wind speeds than shorter blades. The operating wind speed range is considerably narrower than a similar unit running shorter blades. The same longer blade that increases torque now works against the system by increased blade tip speed.
Picture two typical three-blade props from the front. The prop on the left is running 50-foot blades while the prop on the right is running 100 foot blades. Both props are turning at the same shaft speed but the tip speed at the circumference of their arc is much greater on the longer blades. Once the tip speed exceeds the speed of sound the blades both loose effectiveness and become very unstable. While factors such as temperature and humidity influence the actual performance, the tip speed cannot exceed 1130 feet per second. The typical systems in use today are forced to shut down at wind speeds that exceed this limit.